The present invention relates to a sedimentation acceleration agent for activated sludge. More particularly, the present invention relates to an activated sludge sedimentation accelerating agent which can concentrate sedimented sludge in a sedimentation chamber of an activated sludge processing system. The present invention also relates to a method for processing waste water containing activated sludge which uses the sedimentation accelerating agent of the present invention.
Waste water, such as sewage, has been conventionally purified using an activated sludge processing system. The main components of the activated sludge processing system are an aeration chamber and a sedimentation chamber. In this system, waste water is first brought to an aeration chamber where activated sludge is stored. Activated sludge is an aggregate of various microorganisms. The waste water is aerated and mixed with the activated sludge. Organic matter in the waste water is biologically oxidized and broken down. A portion of this oxidized organic matter is converted to activated sludge (microorganism biomass), and another portion of the oxidized organic matter is broken down to carbon dioxide gas and water. The activated sludge forms flocculate clumps ("flocs"), and floats in the mixed liquor.
Next, the mixed liquor in the aeration chamber is transported to the sedimentation chamber. Natural sedimentation of activated sludge by the action of gravity occurs, producing a solid-liquid separation of the above mixed liquor into a supernatant and sedimented sludge. A portion of the sedimented sludge is returned to the aeration chamber as return sludge and is recycled in order to maintain the concentration of activated sludge in the aeration chamber at a set value.
The main drawback of the conventional activated sludge processing system described above is that organic matter in the waste water is not efficiently converted to activated sludge in the aeration chamber. Furthermore, solid-liquid separation of the mixed liquor into sedimented sludge and supernatant in the sedimentation chamber occurs slowly. As a result, the concentration of sedimented sludge is low.
Furthermore, if the sedimented sludge is at a low concentration, the amount that must be returned to the aeration chamber to maintain the concentration of activated sludge in the aeration chamber at a set value becomes large. This results in a shortened aeration time used for purifying the water, and leads to inadequate processing. Therefore, to maximize the operating efficiency of the aeration chamber, the sedimented sludge must be highly concentrated. In other words, the suspended solid (SS) concentration in the mixed liquor (ML) which is to be returned to the aeration chamber (hereinafter "the MLSS") must be increased.
The difference in specific gravity between activated sludge and water is small. Therefore, natural sedimentation of activated sludge in the sedimentation chamber occurs slowly. At times, bulking occurs, and sedimentation no longer occurs. In addition, even in the absence of bulking, the sedimented sludge obtained by natural sedimentation is not highly concentrated.
Furthermore, if organic matter or the like is localized in the waste water, clumped flocs are not generated. Instead, filamentous bulking masses are formed during the aeration step. The sludge is no longer sedimented in the sedimentation chamber. When this occurs, not only is return sludge not generated, but sludge may overflow from the sedimentation chamber.
Several solutions to these problems have been proposed. In Japanese Examined Patent Number 63-59759, an iron or nickel metal powder is added to the aeration chamber. The iron or nickel metal powder is stirred and retained in the activated sludge. By having the activated sludge then attach magnetically to a rotating magnet, the activated sludge and processed water are forced to separate. The activated sludge which is magnetically attached to the rotating body is removed, and is used as the return sludge. A similar method is disclosed in Japanese Laid-Open Patent Number 8-168790.
In Japanese Examined Patent Number 5-10997, sludge sedimentation accelerating agents which have a specific gravity greater than 1 and which coprecipitate with the activated sludge are added to the activated sludge suspension liquor, and the activated sludge and processed water are thereby separated. Examples of such sludge sedimentation accelerating agents include glass powder, alumina powder, and powderized iron oxide.